The present invention relates to water heaters, hot water boilers and air furnaces for residential and commercial use, hereafter referred as residential heaters, which utilize a heat of fuel combustion as a heat source.
The present invention also relates to a method of transmitting a fuel heat energy to a medium.
Residential heaters have found broad implementation both in North America and around the world. That is why they became one of the important issues to cope with greenhouse effect and environmental pollution. For instance, in the United States the adverse impact of the space heating systems is of the same order as the yearly impact generated by highway traffic in conjunction with the most hazardous pollutant formation, such as nitrogen oxides. On the other hand their contribution to the greenhouse effect occurs due to relatively low annual fuel utilization efficiency. The later Department of Energy standard as of 1992 require the annual fuel utilization efficiency to be not less than 80% for hot water and steam residential boilers, and 78% for air heaters and furnaces with output rated up to 300,000 Btu/Hr. Nowadays the best residential boilers available on the market have the annual fuel utilization efficiency up to 84.5% for non-condensing and 91% for condensing designs.
The residential heaters utilize the heat of fuel combustion products to heat up an actuating medium such as water or air in order to bring the necessary amount of heat into a residential heating system. The heat from the combustion product is transferred to an actuating medium through the solid heat exchange surface by means of a free or forced convection. The free heat transfer mode is less efficient especially for gaseous media, i.e. it has an intrinsically low rate of heat transfer coefficient per unit of hour, heat exchange surface, and per a degree of temperature difference. This restricts the efficiency of the residential fuel heaters and, in turn, requires relatively a big and expensive heat exchanger surface to satisfy the necessary heat output. The forced convection which commonly is achieved by means of induced draft, does not make a difference for such restriction and yet leads to more expensive and complicated design.
Another narrow spot of the existing residential heater designs is an unstable and incomplete combustion and a safe operation for those heaters which have the output rated less than 60,000 Btu/Hr. In turn, it mandates the implementation of induced draft which unavoidably involves the above mentioned disadvantages. Besides, with the present design concept the heat transfer occurs at a relatively high average temperature and it causes the thermal nitrogen oxide formation which rising exponentially with the temperature.
Among other related disadvantages of the existing residential heaters involve insufficient safety of their operation. An additional precaution should be exercised to prevent a water hammering in case of water used as the actuating medium. Since water has the boiling temperature much lower than combustion products, a high chance of its sublimation exists. In connection with this, the control is provided with a pressure relief valve to reduce sudden pressure fluctuations of the actuating medium. This again contributes to complications of the control equipment and increase of the overall cost.